Prior art has taught the use of gratings in SiO.sub.2 on Si integrated optical structures to deflect the guided optical waves, focus them, and confine waves to desired optical circuit regions. Also, prior art has taught the fabrication of these gratings by holographic exposure of photoresist layers on the integrated optic surface and sometimes subsequent etching of the waveguide or other optical layers on top of the waveguide or below the waveguide to generate the guided wave-grating interaction.
The development program of a low loss optical waveguide by using the natural oxide of a polished silicon wafer as the beginning material has evolved along two routes or two stages. The first stage of development pursued the route directed to the determination of SiO.sub.2 formation rates on polished Si wafers, and the effects encountered as long, high-temperature processing is performed. The second stage of development pursued the route directed to the treatment of the SiO.sub.2 layer in order to increase the upper film refractive index and to increase the wave binding to the top of the film and avoid the absorption incurred when the evanescent tail of the bound mode extends into the Si substrate.
The growth of SiO.sub.2 substrate is carried out by holding the Si wafer temperature between 900.degree.-1050.degree. C. in steam or O.sub.2 at atmospheric pressure. The oxide grown is roughly twice the thickness of the Si material used. The growth rate of the steam oxide system indicates that an oxide thickness in micrometers (.mu.m) of 2, 3, 7, and 12 require growth times of 10, 30, 100, and 300 hours at 1 atmosphere H.sub.2 O. This system of growth is for (100) oriented Si with slight (10.sup.14) boron doping. This growth rate is greatly enhanced by increasing the steam pressure. At 10 times atmospheric pressure, the oxide formation rate is about five times the one atmosphere rate. The growth rate of SiO.sub.2 is dependent on all the parameters of temperature pressure, orientation, resistivity, and so forth as taught by C. P. Ho and J. D. Plummer: Journal of Electrochemical Society, Vol. 126, No. 9, pp. 1516-1530.
The effect of creating a thick oxide of 5-15 .mu.m on a Si wafer introduces tremendous strains at the interface of the film when the oxidized wafer is cooled to room temperature. This effect is evaluated by reflection techniques wherein a three-inch-diameter wafer with a 14 .mu.m oxide is employed to reflect the image of a white ruler. The strong curvature is evident. This curvature is caused by the different oxide thickness on the polished front versus the unpolished (fine ground) reverse side of the wafer. For oxide thicknesses of less than 8 .mu.m, this deformation has not been observed.
Other deformations of the Si wafer are observed with all the oxide thicknesses, however; and they are even more difficult to eliminate and are certainly more important to the future generation of low loss waveguides on Si subtrates. The crystalline imperfections, dislocations, etc., will propagate when the temperature is high, and the stresses due to the oxide may contribute to this process. These imperfections move from the outer region of the crystal where the growth process has produced them, to the center region of the wafer. This causes the center region (.about.1" diam) of the wafer to appear nonuniform and undulated, and the inclination to spontaneous cleavage in undesirable places is a danger to the wafer. This effect is less of a problem when the substrate resistivity is low (&lt;1.OMEGA.-cm).
The as-grown thick SiO.sub.2 has built-in wave binding condition such that the undoped oxide is a low-scattering, relatively low-loss (.about.3 dB/cm) waveguide. This factor has not yet been explained but the cause of this inherent waveguide is suspected to be related to the thermal expansion coefficient difference between Si and SiO.sub.2 and the large stresses introduced in the film. Also, the SiO.sub.2 density varies with thickness and this contributes to the index profile. The mode index and mode depth versus oxide thickness has been evaluated for the steam oxide waveguides. Clearly the refractive index of the thermally grown SiO.sub.2 does not vary a lot since the TE.sub.0 mode index is about 0.008 greater than the value for thin (&lt;1 .mu.m) thermal oxide SiO.sub.2 layers, and the mode depth is a significant fraction of the SiO.sub.2 film thickness. Also, the linear relationship between mode thickness and SiO.sub.2 total thickness suggests the mechanism generating the index differential is held near the Si--SiO.sub.2 interface.
Most waveguides have surface scattering as the dominate loss mechanism; therefore, if the dominate cause of loss can be identified and corrective procedure applied, a low-scattering waveguide may be developed to have an ultra-low loss. The loss is observed to decrease with increasing oxide thickness where more energy propagates in the low-loss SiO.sub.2 further away from the absorbing Si substrate. This conclusion is reasoned from the observed increase of mode depth with increasing oxide thickness. Thus the preceding argument indicates that the dominate mechanism for loss in the thick oxide waveguides is silicon absorption.
It is clear, therefore, that the thermal oxide waveguide is somewhat unsatisfactory, or at least not an optimum selection to the need for a low-loss waveguide. The second stage of the waveguide development process was pursued with the goal of treating the oxide surface in such a way as to increase the refractive index near the surface and bind the wave more closely to upper surface away from the Si substrate.
Techniques applied by others to form optical waveguides were considered in the plan to reduce the loss in the thick SiO.sub.2 waveguides. Facilities employed such techniques as sputtering and evaporation coating equipment, high temperature diffusion furnaces and ion implantation to 500 kV. Materials employed as surface treatment components included phosphorous, boron, 7059 glass, lead oxide, copper oxide, alumina, and titanium oxide. The implanter was used to dope the SiO.sub.2 with boron and phosphorous, and the other materials were used as diffusant sources in thin film layers on the thermal oxide film.
Efforts pursued by the described techniques have not achieved the desired ultra-low loss optical waveguide. Because of the imperfections of the Si substrate (crystalline imperfections, dislocations, etc., which will propagate when the temperature is high and the stresses due to different oxide thickness on the polished front versus the unpolished, fine ground, reverse side of the wafer), and because of the difficulty in handling the oxidized wafers and, in particular, this difficulty which relates to the fragile nature of the SiO.sub.2 surface, an improved method is desired for producing more perfect gratings in a waveguide requiring low scattering losses. This desired method is enhanced by failure of two techniques pursued wherein the grating is required to be coupled to the waveguide mode either as spatial modulation of the waveguide upper or lower boundary, or as a film deposited upon the upper boundary. Although both techniques were pursued, with the primary effort being devoted to the corregated surface gratings, the thin film gratings of photoresist did not provide the low scattering grating required for a desired device.
Gratings established in SiO.sub.2 waveguide by ion milled technique with a step height of the grating of about 1000 .ANG. proved to be unsuitable for efficient beam reflection due to the nature of the edge of the strip. The roughness of the edge caused intense scattering of the guided wave beam out of the waveguide. Some mode coupling is expected due to the thickness step, but the roughness of several microns is far too coarse for low loss devices.
Therefore, an object of this invention is to provide a method of growing a waveguide of SiO.sub.2 wherein the SiO.sub.2 is produced as a replica of an original Si surface.
A further object of this invention is to produce an integrated optical grating device by a method wherein a grating pattern is first holographically produced on an original highly polished, Si surface, the Si surface pattern is milled away by reactive ion etching, chemical etching, or ion beam milling, and a SiO.sub.2 formation process is completed to grow a 4-8 .mu.m oxide layer which produces the grating replicated in SiO.sub.2.